Semiconductor photoconducting device



' Feb- 16, 1954 JACQUES LPANTcHEcHNlKor-F 2,669,663

Now BY CHANGE orf-NAME JACQUES ISAAC PANKOVE SEMICONDUCTOR PHOTOCONDUCTING DEVICE Filed Nov. 30, 1951 lATTORNEY Patented Feb. 16, M1954 2,669,663 sEMrooNnUo'roR rHo'roooNDUoTING Y 'DEvIoE- Jacques I. Pantehechniko, lnow by change of name Jacques Isaac Pankove, Princeton, N. J.,

"assigner 'to Radio Cor v l poration of America, a Ycorporation 'ofvDelaware Application November 30, 1951, Serial N o` 259,233

conducting material such as highly puried germanium and depositing a thin, light-transmitting lm of any one of a numberof metals on the etched surface. The metals that have been found to give best results for the light-transmitting lm are gold, copper, and nickel. These have previously been disclosed in a'copending .applis cation of Jacques Pantchechnikoi, Serial No. 231,195, filed June 12, 1951.

It has now been found that a device having excellent photoconducting properties can be made by using lms of antiinony, bismuth, or beryllium and applying a bias voltage between the lm and the body of semi-conducting material.l

One object of the present invention is to provide an improved device which includes `a photoconducting cell.

Another object of the invention is to provide a photoconducting device which is relatively simple in construction.

Another object of the invention is to provide c an improved photoconducting device which is rugged in construction and stable in operational characteristics.

Another object of the invention is to provide animprcved photodevice having photoconducting properties when a bias voltage is employed.

I `These and other objects will be more apparent and the invention will be more readily understood from the folios-.ving detail description and the accompanying drawing of which:

Figure 1 is an elevation cross-section view of one form of photocell that can be used in the device of the present invention,

E Figure 2 lis a schematic diagram of a circuit arrangement used with a photocell such as shown in'liigure 1 to make up a device in accordance With the present invention,

Figure 3 is a graph having curves to show comparison between currents in the dark and in the light, with different bias voltages applied,

in a device of the present invention, and

Figure 4 is a view similar to that of Figure A1,-

'7 oisans. (o1. 25e- 211) embodiment of a device constructed inaccordance with the principles of the present invention comprises a photocell which includes a body 2 of germanium semi-conducting material embedded in a block 4 of material such as polystyrene o rother good insulating material. The body of semi-conducting material is embedded in a manner such that one of its faces 6 is exposed. To another surface 8 of the semi-conducting body an electrode Ill is soldered. This electrode may be a nickel stud, for example.

A second electrode l2 isprovided in the kform of a metal cylinder surrounding and concentric with the semi-conductor body 2 and the stud l0. The inner walls of the cylinder are spaced from the semi-conductor body.

A thin, light-transmitting lm of metal I 4 is deposited on the exposed face 6 of the semi-conducting body and across the end of the block of insulating material to the cylindrically shaped electrode I2. In this case the metal should preferably be beryllium, antimony or bismuth for it has been found that when any of these metals is used, the cell acquires photoconducting properties in the presence of a biasing voltage, It has also been found that cells including any of these three metals as the .light-transmitting vnlm do not exhibit photovoltaic properties. The metal may be deposited by well known vapor deposition techniques in vacuo. The thickness of the metal coating may vary considerably but is preferably such that about 10 to 40% of the incident light is transmitted. Ohmic resistance of the lm should preferably be about 500 ohms v per square.

As previously disclosed in the aforementioned co-pending application Serial No. 231,195 it is advantageous to provide a contact electrode in order to provide a better electrical path between that part of the metal nlm Hl overlying the semi-. conductor body i and the end of the cylindrical electrode I2. The contact electrode may comprise a ring iii of silver paste deposited around the edge of the metal film ii such that it overlaps vboth the semi-conductor body and the cylindrical electrode.

Before depositing the metal film on the exposed surface of the semi-conductor body, this surface should be ground smooth and etched. Etching may be carried out using any one of a number of conventional etching solutions which have now become Well known for preparing ger-V manium surfaces to he used inrectiers or ampliiiers; One example of such a solution com# prises' 4 parts by volume of' hydrofluoric acid rst lm. This film may also be applied by deposition from metal vapor, the metal having been vaporized in vacuo. The combined film thickness is preferably such as to permit about 30-40% light transmission. This type of composite lm hasv higher electrical conductivity than one of the same thickness composed entirely of antimony, bismuth, or beryllium.

is used, the germanium must, of course, he highly puried such that it has considerably less than 1% residual impurities and may be made either N- or P-type by doping with the necessary elements in very small amounts. For example, to obtain N-type germanium, tin or arsenic may be added as a donor and, to'obtain P-type germanium, aluminum may be used as the additive.

Referring to Figure 2, if N-type germanium is used, a source of D.-C. biasing voltage is connected across the electrodes l!) and l2 such that the surface film lli is negative with respect to the semi-conductor body 2. The source of bias'-4 ing voltage may comprise a low voltage battery I8. f

The device may also include a pair of output leads 20 and 22 connected to electrodes lil and [2 respectively. An output impedance 24 may be connected across these leads.

In order to utilize the device of the present invention, the output leads are connected to any suitable utilization circuit, such as a light-indicating meter, for example, and light is directed through the central portion of the light-transmitting nlm overlying the semi-conductor body, as indicated by the arrows of Figure 2. As shown in Figure 3, when using a typical cell prepared as described, as the biasing voltage is increased from about 0.1 of a volt to about 1.5 volts, current in the forward direction Ais considerably increased for the illuminated condition (curve A) as compared to the dark condition (curve B). Magnitude of forward current when light is applied depends upon the area illuminated and the internal resistance of the photocell.

Example A cell of the type shown in Figure l was made up using a film of antimony on a body of N-type germanium semi-conducting material. The thickness of the antimony lrnwas such that about 11% of the incident light was transs mitted. Resistance of the nlm was 160 ohms per square. Using a 40 watt, 110 volt Mazda lamp the was positioned so that it was 1 1 cm. from the center of the bulb. This cell delivered 0.3 milliamp. current per sq. mm. area of surface when the bias voltage was about 0.5 volt.

' To provide increased sensitivity, the device of the present invention may be modified by using a slightly diiferent type of photocell. Referring to Figure 4, the modiiied photocell may :comprise the same general parts, i.Y e., a body 2 of semi-conducting germanium embedded in aV block of synthetic resin il, one electrode consisting of a nickel stud i0 soldered to one surface of the semi-conductor body, and second electrode l2 of cylindrical form surrounding but not touching the semi-conductor. En this case, however, the film 26 of antimony, bismuth, or beryllium is much thinner than in the rst embodiment de- It may be made so thin as to transmit -Of the three elements antimony, bismuth, and beryllium, which. have been found to produce photoconoluctive type cells when deposited on a barrier layer type semi-conductor such as germanium, antimony and bismuth are preferred. Photoconductivity is relatively weaker in the case of beryllium. By barrier layer type semi-conductor is meant a semi-conducting material capable of forming a barrier layer at a surface thereof.

Various modifications can be made in the devices which have been described without departing from the spirit of the invention.-For example, any convenient arrangement of electrodes may be utilized, that illustrated being merely one formwhich has been found both simple and eicient.

Instead of germanium any other surface barrier layer forming semi-conductor, such as silicon, may be used.

In connecting up the photocell with the source of bias voltage, if P-type semi-conducting material is used, the metal nlm should be -lwith respect to the semi-conductor body.

Any good insulating material that can be molded readily can be used as the medium in which the semi-conductor is embedded. Examples of suitable materials are polystyrene, cellulose esters and ethers, vinyl resins and acrylic acid derivative resins. It is also possible to omit the embedding material entirely since it does not contribute to the photoelectric properties of the device. If the embedding material is omitted, the metal lm is deposited on the prepared surface of the semi-conductor body and I contact made to the film with a wire.

What is claimed is:

1. An electrical device comprising a body of semi-conducting material capable of forming a barrier layer at a surface thereof and having onv said surface a semi-transparent film of a metal from the class consisting of antimony, bismuth, and beryllium, an electrode in Contact with said body, another electrode in contact with said nlm, a source of bias voltage and means connecting said source across said electrodes.

2. A device according to claim 1 in which said semi-conducting material is germanium.

3. An electrical device comprising a body of N-type semi-conducting material having on a surface thereof a light transmitting film of a metal from theA class Aconsisting of 'antimonia bismuth, and beryllium, an electrode in contact with said body, another electrode in contact with said iilm, a source of bias voltage, and means connecting said source to said electrodes such that said iilm is negative with respect to said body.

4. An electrical device comprising a body of semi-conducting material capable of forming a barrier layer at a' surface thereof, a light-transmitting iilm of a metal from the class consisting of antimony, bismuth, and beryllium on said surface of said material, and means for applying a bias voltage between said body and said film.

5: An electrical device comprising a body of semi-conducting material having on a surface thereof a first semi-transparent lm of a metal from the class consisting of antimony, bismuth, and beryllium, a second semi-transparent lm of a metal from the class consisting `of nickel and copper superimposed on said rst film, an electrode in contact with said body, and another electrode in contact with said second lm.

6. A device according to claim 7 in which said semi-conducting material is germanium.

7. An electrical device comprising a body of semi-conducting material having on a surface thereof a first semi-transparent film of a metal from the class consisting of antmony, bismuth, and beryllium, a second semi-transparent film of 6 a metal from the class consisting of nickel and copper superimposed on said rst film, and means for applying a bias voltage between said body and said films.

JACQUES I. PANTCHECHNIKOFF.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date I0 2,416,604 Wrobel Feb. 25, 1947 2,482,980 Kallmann Sept. 27, 1949 2,496,432 Blackburn Feb. 7, 1950 2,504,628 Benzer Apr. 18, 1950 

1. AN ELECTRICAL DEVICE COMPRISING A BODY OF SEMI-CONDUCTING MATERIAL CAPABLE OF FORMING A BARRIER LAYER AT A SURFACE THEREOF AND HAVING ON SAID SURFACE A SEMI-TRANSPARENT FILM OF A METAL FROM THE CLASS CONSISTING OF ANTIMONY, BISMUTH, AND BERYLLIUM, AN ELECTRODE IN CONTACT WITH SAID BODY, ANOTHER ELECTRODE IN CONTACT WITH SAID FILM, A SOURCE OF BIAS VOLTAGE AND MEANS CONNECTING SAID SOURCE ACROSS SAID ELECTRODES. 